Connected: An Internet Encyclopedia
RFC 791 - IP Header Option: Internet Timestamp

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### RFC 791 - IP Header Option: Internet Timestamp

            +--------+--------+--------+--------+
            |01000100| length | pointer|oflw|flg|
            +--------+--------+--------+--------+
            |         internet address          |
            +--------+--------+--------+--------+
            |             timestamp             |
            +--------+--------+--------+--------+
            |                 .                 |
                              .
                              .
            Type = 68

The Option Length is the number of octets in the option counting the type, length, pointer, and overflow/flag octets (maximum length 40).

The Pointer is the number of octets from the beginning of this option to the end of timestamps plus one (i.e., it points to the octet beginning the space for next timestamp). The smallest legal value is 5. The timestamp area is full when the pointer is greater than the length.

The Overflow (oflw) [4 bits] is the number of IP modules that cannot register timestamps due to lack of space.

The Flag (flg) [4 bits] values are

0 -- time stamps only, stored in consecutive 32-bit words,

1 -- each timestamp is preceded with internet address of the registering entity,

3 -- the internet address fields are prespecified. An IP module only registers its timestamp if it matches its own address with the next specified internet address.

The Timestamp is a right-justified, 32-bit timestamp in milliseconds since midnight UT. If the time is not available in milliseconds or cannot be provided with respect to midnight UT then any time may be inserted as a timestamp provided the high order bit of the timestamp field is set to one to indicate the use of a non-standard value.

The originating host must compose this option with a large enough timestamp data area to hold all the timestamp information expected. The size of the option does not change due to adding timestamps. The intitial contents of the timestamp data area must be zero or internet address/zero pairs.

If the timestamp data area is already full (the pointer exceeds the length) the datagram is forwarded without inserting the timestamp, but the overflow count is incremented by one.

If there is some room but not enough room for a full timestamp to be inserted, or the overflow count itself overflows, the original datagram is considered to be in error and is discarded. In either case an ICMP parameter problem message may be sent to the source host [3].

The timestamp option is not copied upon fragmentation. It is carried in the first fragment. Appears at most once in a datagram.

Connected: An Internet Encyclopedia
RFC 791 - IP Header Option: Internet Timestamp